Apparatus and method of making precision metal spheres

ABSTRACT

A method of forming metal spheres includes ejecting a precisely measured droplet of molten metal from a molten metal mass, buffering the molten metal droplet to reduce the internal kinetic energy of the droplet without solidifying the droplet and cooling the buffered droplet until the droplet solidifies in the form of a metal sphere. An apparatus for fabricating metal spheres includes a droplet generator that generates a droplet from a molten metal mass, a buffering chamber that receives the droplet from the droplet generator, and diminishes internal kinetic energy of the droplet without solidifying the droplet, and a cooling drum that receives the droplet from the buffering chamber, and cools the droplet to the extent that the droplet solidifies into a metal sphere. The apparatus may further include a collector arrangement that receives the metal spheres from the cooling drum and makes the metal sphere available for collection.

FIELD OF THE INVENTION

[0001] The present invention relates to methods of making metal spheres.In particular, the present invention relates to making metal spheresfrom molten metal, such that the solid metal spheres achieve a veryclose tolerance for sphericity and size. Such metal spheres,particularly precision miniature metal spheres, have many industrialapplications. For example, such spheres may be used to form Ball GridArray (BGA) and Flip Chip (FC) arrangements in high-density integratedcircuit packaging, and are also used as writing tips of ball pens.

BACKGROUND OF THE INVENTION

[0002] Conventionally, small precision metal spheres are made using amechanical process by which a number of small metal particles are cut orpunched out from fine wire or sheets. Those particles are then droppedinto a tank of hot oil having a temperature that is higher than that ofthe melting point of the particles. In this hot oil bath, all the metalparticles are melted, forming small round droplets due to surfacetension of the molten metal. As the temperature of the oil cools down tobelow the melting point of the metal droplets, the droplets solidifyinto spheres. This mechanical method has intrinsic limitations thatresult in coarse dimensional tolerances, because each mechanicaloperation adds a certain amount of deviation to the size and uniformityof the particles. which together produce an unacceptable cumulativeeffect. Therefore, spheres are not precisely made according to thisprocess. Further, the resulting spheres must undergo a sophisticatedwashing process to get rid of the oil and other surface contaminants.

[0003] Over the past two decades, many methods have been developed forgenerating precision molten droplets to improve the dimensionaltolerances of the spheres. These new methods commonly utilize a cruciblein which to melt the metal, and then cause the molten metal to flow outof the crucible through a small nozzle. Droplets are formed by shakingeither the crucible or the nozzle, or by oscillating inlet gas to affectthe pressure on the molten metal in the crucible. These types ofvibratory disturbances that are used to generate the droplets aretypically controlled by some electronic means. Due to the surfacetension of the molten metal droplets, they automatically form aspherical shape while passing through a cooling medium after passingthrough the nozzle. However, the parameters of those processes and theenvironmental conditions of the electronic droplet generators arecritical to the uniformity of the output. In many cases, these processescan only reach a quasi-steady-state, which limits the productionthroughput as well as the quality of the resulting spheres.

[0004] There is therefore a need for a process for forming metal spheresby which tolerances on the size and shape of the spheres can be keptsmall. Such a process must allow for a reasonable throughput andprocessing of the spheres such as by washing and other finishing actionsshould be kept to a minimum. In order to be truly useful, such a processmust relatively simple, requiring few controls of parameters of theprocess.

SUMMARY OF THE INVENTION

[0005] It is therefore an objective of the present invention to providea process by which precision metal spheres may be formed.

[0006] It is a further objective of the present invention to provide aprocess by which the degree of deviation from a perfect spherical shapeof the metal spheres can be minimized.

[0007] It is an additional objective of the present invention to providea process by which the size of the metal spheres can be determinedwithin a small tolerance.

[0008] It is also an objective of the present invention to provide aprocess by which metal spheres are formed such that the metal spheresrequire less post-formation cleaning than do conventionally-producedmetal spheres.

[0009] It is another objective of the present invention to provide aprocess by which fewer parameters must be controlled than when utilizingconventional processes.

[0010] It is a further objective of the present invention to provide aprocess by which throughput of the metal spheres is not hampered by theprecision achieved in the finished product.

[0011] It is also an objective of the present invention to provide anapparatus that facilitates the process of the present invention.

[0012] The present invention is a method of forming metal spheres frommolten metal in which precisely-sized droplets of the molten metal areseparated from a metal mass to form the metal spheres. The droplets ofthe molten metal are first projected in an upward direction and bufferedprior to descending through a cooling medium. Through the use of inletgas and liquid, the cooling medium is controlled for precisionsolidification of the metal spheres. The solid spheres enter a liquidbath in a collection receptacle at the end of the cooling process, wherethey are automatically collected and separated from the liquid, which isreturned to the collection receptacle for reuse.

[0013] Instead of disturbing the steady flow of the molten metal streamto create droplets, the method of the present invention utilizes a fastvibratory piston to strike each individual droplet out through a nozzle.Driven in this manner, the droplets can be shot initially upward througha cooling medium and spend more time passing through the medium beforesolidification of each droplet begins. Thus, a shorter cooling tower canbe used, thereby saving costs related to the height of the manufacturingroom, as well as reducing the amount of coolant required during thesolidification process. As the piston slams a stopper or withdraws itsdirection of motion quickly, the resulting sudden impact transfers theenergy at the piston to the molten metal and creates a droplet thatshoots out through the nozzle. Control of the striking force of thepiston against the stopper, and knowledge of the size of the aperture inthe nozzle, allow droplets of molten metal having precisely-controlledvolumes to be separated from the molten metal mass and propelled throughthe cooling medium, allowing for the formation of spheres of uniformsize.

[0014] The structure of the apparatus of the present invention includesa buffering chamber that is designed to provide the cooling dropletswith enough time to allow the internal energy to settle down beforefinal formation and solidification. The kinetic energy within a moltendroplet is usually higher than its surface tension energy right afterthe droplet changes dynamically in this fashion, and therefore thedroplet does not acquire a spherical shape until a large percentage ofthis internal kinetic energy is released. When the surface tension of adroplet dominates the internal kinetic energy as the molten metal cools,the shape of the droplet becomes spherical automatically. As previouslystated, the molten metal droplets are first propelled in an upwarddirection in the chamber, before being overcome by gravity and allowedto fall back downward. This buffering chamber has a heating system thatcontrols the temperature of the gas inside the chamber to prevent thedroplets from solidifying before the shape of the sphere is mature. Thegas used is preferably an inert gas such as nitrogen, or a mixture ofnitrogen and hydrogen. The temperature inside the chamber is determinedempirically, depending on certain properties of the molten droplets.Typically, this temperature falls in the range between 0° C. and 100°C., depending on the size and material of the droplets.

[0015] A gas screen gate is disposed beneath the buffering chamber. Thisgate is a large hollow disc with two openings, one each at the centersof both top and bottom faces of the circular disc. One or more fans aredisposed inside the disc along the edge of the disc wall. The fan blowsin a direction tangential to the circular wall, causing the gas withinthe disc to flow in a circular direction within the hollow interior ofthe disc. This movement creates a gas barrier that slows down the heatexchange rate between the buffer chamber and the top end of the coolingtower, so that the droplets do not experience quick cooling while stillin the buffering chamber. The two openings in the gate allow thedroplets to pass out of the buffering chamber under the force ofgravity.

[0016] Below the gas gate, a number of cooling drums are connected in astack to form a cooling tower. Each drum has two sections formed bycoaxial cylinders. The inner section of the drum is a cylinder having anopen top and bottom so that the falling droplets can pass through. Anouter shell forms a container with the cylindrical wall of the innersection, and is used to hold coolant or other low temperature agent suchas liquid nitrogen. There are two small inlet pipes connected to theouter container of the drum. One is used to provide coolant to the outercontainer, and the other is used to blow a cold agent or low temperaturegas around the inner section when rapid cooling is required. There are anumber of small openings around the top part of the wall separating theinner section from the outer shell, to relieve pressure on thecylindrical walls and provide a passage for additional inert gas to beprovided to the cooling tower.

[0017] At the bottom of the cooling tower, there is a funnel shapedcollector. The collector has an outer hollow shell that is pumped intovacuum to provide good thermal insulation. The collector is filled witha liquid cooling agent such as Hexane, which has a melting point ofabout −100° C. The liquid agent also serves to provide a low-impactmedium that stops the falling metal spheres. At the termination of thecollector, there is a collecting container used to collect the mixtureof solidified spheres and cooling liquid. This mixture is pumped up toabove the liquid level of the collector and then flows downward into thecollecting container, in which is placed a fine mash basket. Thecontainer has a pipe at the bottom end to allow the liquid to flow backto the collector after the mesh basket catches the metal spheres. Thespheres that are trapped in the mesh basket can then be collected, suchas by picking them out through the top opening of the container. Thecontainer opening has a gas-tight door, and the feedback pipe has avalve to prevent backflow.

[0018] In summary, a method of forming metal spheres according to thepresent invention includes ejecting a precisely measured droplet ofmolten metal from a molten metal mass, buffering the molten metaldroplet to reduce the internal kinetic energy of the droplet withoutsolidifying the droplet and cooling the buffered droplet until thedroplet solidifies in the form of a metal sphere. The method may alsoinclude collecting the metal sphere.

[0019] Ejecting a droplet of molten metal may include disposing themolten metal mass in a fixed volume, providing an aperture as an outletto the fixed volume, striking the molten metal mass with an impulseforce and allowing the impulse force to propagate through the moltenmetal mass to cause a droplet of the molten metal mass to be ejectedthrough the aperture. Preferably, the droplet is ejected in a generallyupward direction.

[0020] Buffering the molten metal droplet may include cooling thedroplet to an extent that is less than is necessary to cause the dropletto solidify, and allowing internal kinetic energy of the droplet todiminish. Further, buffering the molten metal droplet may includeallowing the ejected droplet to ascend to a maximum height, and thenallowing the droplet to descend through a medium having a temperaturethat is controlled such that the droplet is cooled but not allowed tosolidify.

[0021] Cooling the buffered droplet may include allowing the droplet todescend through a medium having a temperature that is controlled to coolthe droplet.

[0022] Collecting the metal sphere may include immersing the metalsphere in a liquid, and separating the metal sphere from the liquid.Separating the metal sphere from the liquid may include depositing theliquid and the metal sphere in a container having drainage holes thatare smaller than the metal sphere, and draining the liquid from thecontainer through the drainage holes.

[0023] An apparatus for fabricating metal spheres according to thepresent invention includes a droplet generator that generates a dropletfrom a molten metal mass, a buffering chamber that receives the dropletfrom the droplet generator, and diminishes internal kinetic energy ofthe droplet without solidifying the droplet, and a cooling drum thatreceives the droplet from the buffering chamber, and cools the dropletto the extent that the droplet solidifies into a metal sphere. Theapparatus may further include a collector arrangement that receives themetal spheres from the cooling drum and makes the metal sphere availablefor collection.

[0024] The droplet generator may include a receptacle in which themolten metal mass is contained, wherein the receptacle includes aplurality of walls and a tube, an aperture through a first wall of theplurality of walls of the receptacle, and a piston disposed within thetube and forming a substantially fluid-tight seal with the tube. Areciprocating motion of the piston within the tube changes pressure ofthe molten metal mass, and an impulse force imparted by the piston onthe molten metal mass within the receptacle causes a portion of themolten metal mass to eject through the aperture as a droplet. Thedroplet generator may also include a feed tube extending outward fromthe aperture; the piston abuts the first wall at an end of thereciprocating motion such that the piston closes off the aperture fromthe inside of the receptacle and forces a droplet of molten metal out ofthe feed tube. The droplet generator may be positioned such that thedroplet is ejected in an upward trajectory.

[0025] The buffering chamber may include an enclosed volume having aheight sufficient to allow the ejected droplet to reach a maximumunimpeded height in the upward trajectory. The buffering chamber mayinclude an enclosed volume containing a gaseous medium, and atemperature control system that controls the temperature of the gaseousmedium. The enclosed volume may include a bottom end having an openingfor receiving the droplet as it descends after reaching the maximumunimpeded height in the upward trajectory.

[0026] The cooling drum may include a first cylinder, having an open topend and an open bottom end and surrounding a gaseous medium, a secondcylinder; coaxial with the first cylinder and surrounding the firstcylinder, and having a top end that is closed around the top end of thefirst cylinder, and a bottom end that is closed around the bottom end ofthe first cylinder, forming a reservoir between the first and secondcylinders, and a system for controlling the temperature of the gaseousmedium.

[0027] The system for controlling the temperature of the gaseous mediummay include a first fluid inlet, disposed in an outer wall of the secondcylinder, that receives a first fluid to be stored in the reservoir, anda second fluid inlet, disposed in the outer wall of the second cylinder,for receiving a second fluid to be dispersed within the first fluid inthe reservoir. The system may also include a dispersal tube, connectedto the second fluid inlet and surrounding the first cylinder within thereservoir, that receives the second fluid through the second fluidinlet, wherein the dispersal tube includes a plurality of holes throughwhich the second fluid is dispersed within the first fluid. Preferably,the dispersal tube is a circular closed loop for receiving the secondfluid from the second fluid inlet and for dispersing the second fluidinto the first fluid, within the reservoir around the first cylinder,through the plurality of holes.

[0028] The apparatus may also include a gas screen disposed between thebuffering chamber and the cooling drum, which provides temperatureseparation between respective media in the buffering chamber and thecooling drum. The gas screen may include a hollow disk having a top facewith an opening for receiving the droplet from the buffering chamber, abottom face with an opening for providing the droplet to the coolingdrum, and circular outer wall connecting the top and bottom faces, and afan, disposed within the hollow disk and positioned such that it blows afluid medium within the hollow disk in a direction that is tangential tothe outer wall.

[0029] The collector arrangement may include a reservoir that holds aliquid into which the metal sphere falls after passing through thecooling drum, a pipe, connected to a bottom end of the reservoir and influid communication with the reservoir, that receives the metal sphereand a volume of the liquid from the reservoir, and a delivery systemthat delivers the metal sphere to a collection basket. The reservoir mayhave lower sides that slope toward an opening in the pipe. The pipe maybe an elbow joint having a bend in which the metal sphere settles. Thedelivery system may be a pump that pumps the metal sphere and the volumeof the liquid to the collection basket, and the collection basket may belocated at a level that is higher than a level of the liquid in thereservoir. The collector arrangement may include a holding tank in whichthe collection basket is disposed, and the collection basket hasopenings that are smaller than the metal sphere, through which thevolume of liquid pass. The collector arrangement may include a returnchannel, in fluid communication between the holding tank and thereservoir, by which liquid passing through the openings in thecollection basket is returned to the reservoir.

[0030] The cooling drum may be a plurality of cooling drums, including afirst cooling drum, disposed to receive the droplet from the bufferingchamber, and a last cooling drum, disposed to provide the metal sphereto the collector arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 shows a sectional diagram of an exemplary apparatus of thepresent invention.

[0032]FIG. 2a shows a first embodiment of a molten metal dropletgenerator of the present invention.

[0033]FIG. 2b shows a second embodiment of a molten metal dropletgenerator of the present invention.

[0034]FIG. 3 shows an exemplary buffering chamber of the presentinvention.

[0035]FIG. 4 shows an exemplary gas screen of the present invention.

[0036]FIG. 5 shows an exemplary cooling drum of the present invention.

[0037]FIG. 6 shows an exemplary metal sphere collection system of thepresent invention.

[0038]FIG. 7 is a flow diagram of the method of the present invention.

[0039]FIG. 8 is a flow diagram of the process of forming droplets of thepresent invention.

[0040]FIG. 9 is a flow diagram of the process of buffering the dropletsof the present invention.

[0041]FIG. 10 is a flow diagram of the process of cooling the dropletsof the present invention.

[0042]FIG. 11 is a flow diagram of the process of collecting the spheresof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The present invention provides a process by which metal spherescan be fabricated. As shown in FIG. 7, the process begins with theformation of molten metal droplets 71. The droplets undergo a bufferingaction 72 to reduce the internal kinetic energy of the droplets prior tofinal cooling of the droplets to a solid form. Once the internal kineticenergy has been reduced a sufficient amount, the cooling process 73 canbegin. Because the internal kinetic energy of the droplets has beenreduced at this point, a droplet will form a spherical shape as itcools, due to the surface tension of the molten metal material. Aftercooling for a sufficient amount of time, the droplets become solidspheres 74, and are collected 75.

[0044] As shown in FIG. 8, the droplets are formed by providing a massof molten metal, and exerting an impulse force to the mass of moltenmetal. The molten metal mass is constrained within a fixed volume 710,which is provided with a single outlet aperture 711. The impulse forcethat is applied to the molten metal mass 712 transmits through themolten metal mass. When this transmission of the impulse force reachesthe surface of the molten metal mass near the aperture, the surfacetension of the molten metal mass is broken there 713. Because thesurface tension is broken, a portion of the metal mass breaks away andis forced out of the volume through the aperture, in the form of adroplet 714. The size of the droplet is determined by the size of theaperture, and the magnitude and duration of the impulse applied to themolten metal mass.

[0045] Once the droplet has been expelled through the aperture in thismanner, its internal kinetic energy is high, and may even dominate thesurface tension of the liquid droplet. Therefore, the buffering actiontakes place at this point as shown in detail in FIG. 9. Buffering takesplace by slowly cooling the droplets. This is accomplished by providingan environment wherein the temperature is kept in a range that will coolthe droplets but not to the extent that they will quickly solidify.Assisting in this buffering process is the motion of the droplets. Whenthe droplet is expelled through the aperture, the force experienced bythe droplet ejects the droplet at great speed. Therefore, the path ofthe ejected droplet is directed generally upward. The droplet is allowedto travel through the buffering medium and gradually slow down in thisgenerally upward trajectory until stopping at a maximum height due tothe effects of gravity 720. The droplet then begins its descent due togravity through the buffering space 721. As described above, the spacein which the droplet descends has a temperature that is controlled 722.The droplet is allowed to fall under these controlled conditions untilthe internal kinetic energy of the droplets has sufficiently diminished723, without causing the droplets to solidify. As described previouslywith reference to FIG. 7, the next process will be to cool the dropletsfurther 73. Thus, part of the buffering process 72 preferably includesproviding a gas screening action 724 between the buffering and coolingprocesses, to provide temperature separation as the droplets pass fromthe buffering stage 72 to the cooling stage 73. This may be effected bysetting up a zone between the buffering medium and the cooling medium,whereby heat exchange between the two mediums is minimized.

[0046] The droplet is then cooled by providing a cooling medium 730through which the falling droplet continues its descent 731. As thedroplet falls through the cooling medium 731, it gradually changes froma molten, liquid state to a solid state, in the shape of a sphere 732.The time spent in the cooling medium must be sufficiently long to enablethe spheres to harden completely. Because the droplets are falling asthey cool, the length of cooling time is determined by the length of thepath that the droplet is allowed to fall during the cooling process.

[0047] After the droplets have completely hardened and have become solidspheres, they must be collected. Further, because the droplets have beenfalling through a cooling medium during the cooling process, the motionof the falling spheres must be stopped 750. This is accomplished byallowing the spheres to plunge into a liquid bath at the termination ofthe cooling path. This liquid bath is a collection medium in which anumber of metal spheres are accumulated 751. This mixture of spheres andmedium is then delivered to a collection space 752, where the spheresare separated from the collection medium 753. The spheres can then becollected 754, and the collection medium preferably can be returned tothe liquid bath 755. This is accomplished by pumping the liquid andsphere mixture from the bottom of the liquid bath up to a level abovethe level of the liquid bath. The liquid and sphere suspension is thendrained such that the spheres are captured and the liquid is returned tothe bath. The captured spheres may then be collected.

[0048]FIG. 1 shows an overall view of the apparatus of the presentinvention. The structure of the invention can be divided into four majorsections. The first section is the droplet generator 1, which producesthe droplets that form the metal spheres. The second section is thebuffering chamber 2, where the propelled droplets reach a peak heightbefore beginning the fall toward the cooling drums, while dissipatinginternal kinetic energy under controlled temperature conditions. Thethird section is the cooling drum 3 a number of which may be providedand stacked in series as necessary. The solid metal spheres are formedas the droplets cool while passing through these drums. The fourthsection is the collector 4, where the solid metal spheres end theirdescent and are gathered for collection.

[0049]FIG. 2a shows an exemplary droplet generator 5 according to thepresent invention. This embodiment of the droplet generator isparticularly advantageous for producing droplets of any size larger thanapproximately 0.1 mm. The molten metal is provided to the inlet 6 of aT-shaped tube 7. The pressure of the liquid metal is controlled suchthat it is balanced with the surface tension of the molten metal at thetop end 8 of the T-shaped tube 7. At this top end 8, there is a smallhole that serves as a nozzle 9. A piston 10 is mounted opposite thenozzle 9 within the bottom end 11 of the T-shaped tube 7. The piston 10provides a substantially airtight seal with the inner wall of the bottomend 11 of the T-shaped tube 7. When the piston moves up and down rapidlywithin the bottom end 11 of the T-shaped tube 7, it breaks the balanceof forces between the surface tension and the pressure in the liquidmetal. That is, the impact force of the piston on the molten metalwithin the T-shaped tube 7 is transmitted through the molten metal tothe surface of the molten metal 12 at the top end 8 of the T-shaped tube7. When this occurs, the internal pressure of the molten metal at thetop end 8 exceeds the surface tension, allowing a portion of the moltenmetal to break away. Because the nozzle 9 is the only aperture throughwhich this portion of the molten metal can escape. each up and downcycle of the piston motion generates a droplet of the molten metalpushed through the nozzle 9 as an output of the T-shaped tube 7. Themotion of the piston 10 is preferably driven electronically, for exampleby an electro-mechanical transducer 13, such as a magnetic coil or piezocrystal, so that it can be controlled for uniform speed, distance ofmovement, and impact force.

[0050]FIG. 2b shows an alternative embodiment of the droplet generator20 of the present invention. This embodiment is particularlyadvantageous for producing droplets of any size between approximately0.10 mm and 2.50 mm. A stopper 21 is added at the front end of thereciprocating piston 22 motion. With each motion of the piston 22, thereis a collision between the piston 22 and stopper 21, which closes offthe proximate opening 23 in the nozzle feed tube 24 leading to thenozzle outlet 25 located at the distal end 26 of the nozzle feed tube24, thereby forcing a droplet of molten metal out of the nozzle outlet25. The piston displacement is very small and precise, and thereforecauses an accurately measured amount of molten metal to be dispelledfrom the nozzle, which in turn becomes a droplet of predetermined sizethat forms a metal sphere having precisely controlled dimensions.

[0051]FIG. 3 shows the structure of a buffering chamber 30 utilized toprovide a space for the droplets to propel up and then fall backdownward in a temperature-controlled environment. The droplet generator31 dispels the droplets in an upward direction, such that they follow apath 32 over a dividing wall 33 before descending over the far side ofthe wall 33. In the area 34 of the chamber on the far side of the wall33, there is an air circulation system 35 that includes a heat exchanger36, which is used to control the temperature of the gas inside the area34. A fan 38 draws air from the area 34 into the heat exchanger 36,where the temperature of the air is adjusted before being expelled backinto the area 34. Usually, the temperature is kept between 25° C. and100° C. As previously explained, the air temperature is kept at a levelthat allows the internal kinetic energy of the droplets in the area 34to gradually dissipate, so that the droplets are better prepared for thecooling stage that will actually solidify the droplets. This bufferingstage prevents the sudden, premature cooling and solidification that canresult in approximate metal spheres having dimensions with unacceptablyeccentric qualities.

[0052] As shown, the chamber 30 has an opening 37, preferably circular,at the bottom of the structure to allow the droplets drop through,leading to a gas screen. The gas screen 40, as shown in FIG. 4, isdesigned to provide temperature insulation between the relatively warmbuffering chamber 30 and the colder drum below. The gas screen is ahollow circular disc structure having a top face 41 adjacent thebuffering chamber 30, a bottom face 42 adjacent the cooling drum below,and a generally circular outer wall 43. The top and bottom faces of thedisc each have an opening 44, 45, which is preferably circular in shape.One or more fans 46 are built inside the disc to direct the gas withinthe gas screen 40 such that it circulates 47 about the center axis ofthe disc. The circular motion of the air acts to prevent heat exchangebetween the air in the buffering chamber 30 above the gas screen and thecooling chamber disposed below the gas screen 40. The droplet, in itstrajectory through the buffering chamber 30, passes through the opening37 in the bottom of the buffering chamber 30, through the upper opening44 in the gas screen 40, through the lower opening 45 in the gas screen40, and into the cooling drum disposed below the gas screen 40.

[0053] At least one such cooling drum 3 is located below the bottom face42 of the gas screen 40, and the gas screen 40 may be disposed atop astack of such cooling drums, as shown in FIG. 1. FIG. 5 shows thestructure of an individual cooling drum 50 in the stack. The number ofsuch cooling drums 50, if used in a stack, depends on the parameters ofthe particular cooling application. Such parameters include the size andmaterial of the metal droplets, the impact of the droplet generator andattendant height reached by the propelled metal droplet, the amount ofbuffering time experienced by the metal droplet, and the height of eachindividual cooling drum 50.

[0054] Each cooling drum 50 includes two coaxial cylinders 51, 52. Theinner cylinder 51 is hollow and has substantially open top 53 and bottom54 ends, so that the droplets can pass through. The outer cylinder 52also has a hollow interior, surrounding the inner cylinder 51, providinga chamber space 55 around the inner cylinder 51. This chamber space 55is closed at top 56 and bottom 57 ends. The inner cylinder 51 also hasat least one and preferably multiple holes 58 in the cylinder wallseparating the inner 51 and outer 52 cylinders, toward the upper end ofthe inner cylinder 51. The outer cylinder 52 also has two inlet ports 58a, 59 a, each connected to a respective feed pipe or tube 58 b, 59 b.The first inlet port and tube 58 a,b are used to add a low temperatureliquid, such as liquid nitrogen, to the chamber space 55 inside theouter cylinder 52 and outside the inner cylinder 51. The first inletport 58 a is located at height that allows the chamber space 55 to befilled sufficiently with the liquid, which acts as the coolant for thecooling drum. The second inlet port and tube 59 a,b are used to providea gas or gas mixture, such as 20% hydrogen in nitrogen, to a ring pipe59 c that is connected to the second inlet tube 59 b and which encirclesthe inner cylinder 51 within the chamber space. The second inlet port 59a, second inlet tube 59 b, and ring pipe 59 c are located below thefirst inlet port 58 a. Thus, when the chamber space 55 is sufficientlyfilled with the coolant liquid. the ring pipe 59 c is submersed in theliquid. After the chamber space 55 is sufficiently filled with thecoolant preferably when the chamber space 55 is approximately halffilled, gas is provided to the ring pipe 59 c through the second inletport 59 a. The ring pipe 59 c has a number of small gas release holes60, through which gas in the ring pipe 59 c is released into the coolantliquid in the chamber space 55. Thus, the temperature inside the coolingdrum 50 is controlled by the temperature of the coolant liquid and alsoby the flow rate of the gas that blows through the liquid. In thismanner, the temperature of the passage within the inner cylinder 51 canbe maintained with a high degree of accuracy, so that a degree ofcontrol can be exercised over the solidification of the metal dropletpassing through this passage. Quickly increasing the flow rate of theinlet gas can also provide rapid cooling of the passage, if necessary.

[0055] Below the cooling drum 50, or below the bottom cooling drum 50 ofthe cooling tower, there is a sphere collecting arrangement 4, as shownin FIG. 1. This arrangement 68, as shown in detail in FIG. 6, includes afunnel-shaped reservoir 61, an elbow pipe or tube structure 62, a drumpump 63, and a collection tank 64. The reservoir 61 is located directlybeneath the cooling drum 50 or tower, and contains a low freezing pointliquid. such as Hexane. As a metal droplet falls from the top end of thefirst cooling drum to the bottom end of the last cooling drum, itsolidifies into a spherical shape, and then plunges into the liquid inthe reservoir 61. The solid metal balls then make their way down theslopes of the sides of the reservoir 61, and collect at the bottom ofthe elbow structure 62. The drum pump 63, which is connected to theother end of the elbow structure 62. pumps the liquid and metal spheremixture up to the collection tank 64, such that all the metal sphereswithin the elbow structure 62 move with the liquid. A mesh basket 65,which is disposed inside the collection tank 64, receives the liquid andmetal sphere mixture from the pump through a channel 66 or the like. Themesh basket 65 separates the solid spheres from the liquid. That is, theopenings in the mesh walls of the basket 65 are smaller than the metalspheres, so that the liquid passes through the mesh walls of the basket65, leaving only the metal spheres behind. The collection tank 64 isconnected to the reservoir 61 by a pipe 67, through which the liquidflows back to the reservoir 61 after the metal spheres have beenseparated by the mesh basket 65. This is possible because the collectiontank 64 is located at a point that is higher in elevation than theliquid level in the reservoir 61, so that the liquid naturally flowsback to the reservoir 61, preventing waste of the reservoir liquid.Therefore, the drum pump 63 must be able to draw the liquid and metalsphere mixture up to the level of the collection tank 64. The entiresphere collecting arrangement 68 is preferably enclosed in a gas-tightcabinet 69 that has a closable opening 70 through which metal spheresthat have accumulated in the mesh basket can be collected.Alternatively, the mesh basket 65 itself can be removed through theopening 70, and replaced with an empty mesh basket 65.

What is claimed is:
 1. A method of forming metal spheres, comprising:ejecting a precisely measured droplet of molten metal from a moltenmetal mass; buffering the molten metal droplet to reduce the internalkinetic energy of the droplet without solidifying the droplet; andcooling the buffered droplet until the droplet solidifies in the form ofa metal sphere.
 2. The method of claim 1, further comprising collectingthe metal sphere.
 3. The method of claim 1, wherein ejecting a dropletof molten metal includes disposing the molten metal mass in a fixedvolume; providing an aperture as an outlet to the fixed volume; strikingthe molten metal mass with an impulse force; and allowing the impulseforce to propagate through the molten metal mass to cause a droplet ofthe molten metal mass to be ejected through the aperture.
 4. The methodof claim 3, wherein the droplet is ejected in a generally upwarddirection.
 5. The method of claim 1, wherein buffering the molten metaldroplet includes cooling the droplet to an extent that is less than isnecessary to cause the droplet to solidify.
 6. The method of claim 1,wherein buffering the molten metal droplet includes allowing internalkinetic energy of the droplet to diminish.
 7. The method of claim 4,wherein buffering the molten metal droplet includes allowing the ejecteddroplet to ascend to a maximum height, and then allowing the droplet todescend through a medium having a temperature that is controlled suchthat the droplet is cooled but not allowed to solidify.
 8. The method ofclaim 1, wherein cooling the buffered droplet includes allowing thedroplet to descend through a medium having a temperature that iscontrolled to cool the droplet.
 9. The method of claim 2, whereincollecting the metal sphere includes immersing the metal sphere in aliquid, and separating the metal sphere from the liquid.
 10. The methodof claim 9, wherein separating the metal sphere from the liquid includesdepositing the liquid and the metal sphere in a container havingdrainage holes that are smaller than the metal sphere, and draining theliquid from the container through the drainage holes.
 11. The method ofclaim 9, wherein the liquid is contained in a reservoir; and the metalsphere is drawn upward with some of the liquid until the metal spherereaches a level that is higher than the level of the liquid in thereservoir.
 12. The method of claim 11, wherein separating the metalsphere from the liquid includes allowing the drawn liquid to flow backdownward to the reservoir.
 13. An apparatus for fabricating metalspheres, comprising: a droplet generator that generates a droplet from amolten metal mass; a buffering chamber that receives the droplet fromthe droplet generator, and diminishes internal kinetic energy of thedroplet without solidifying the droplet; and a cooling drum thatreceives the droplet from the buffering chamber, and cools the dropletto the extent that the droplet solidifies into a metal sphere.
 14. Theapparatus of claim 13 further comprising a collector arrangement thatreceives the metal spheres from the cooling drum and makes the metalsphere available for collection.
 15. The apparatus of claim
 13. whereinthe droplet generator includes a receptacle in which the molten metalmass is contained, wherein the receptacle includes a plurality of wallsand a tube; an aperture through a first wall of the plurality of wallsof the receptacle; and a piston disposed within the tube and forming asubstantially fluid-tight seal with the tube; wherein reciprocatingmotion of the piston within the tube changes pressure of the moltenmetal mass.
 16. The apparatus of claim 15, wherein an impulse forceimparted by the piston on the molten metal mass within the receptaclecauses a portion of the molten metal mass to eject through the apertureas a droplet.
 17. The apparatus of claim 16, wherein the dropletgenerator further includes a feed tube extending outward from theaperture; and the piston abuts the first wall at an end of thereciprocating motion such that the piston closes off the aperture fromthe inside of the receptacle and forces a droplet of molten metal out ofthe feed tube.
 18. The apparatus of claim 16, wherein the dropletgenerator is positioned such that the droplet is ejected in an upwardtrajectory.
 19. The apparatus of claim 18, wherein the buffering chamberincludes an enclosed volume having a height sufficient to allow theejected droplet to reach a maximum unimpeded height in the upwardtrajectory.
 20. The apparatus of claim 13, wherein the buffering chamberincludes an enclosed volume containing a gaseous medium; and atemperature control system that controls the temperature of the gaseousmedium.
 21. The apparatus of claim 19, wherein the enclosed volumeincludes a bottom end having an opening for receiving the droplet as itdescends after reaching the maximum unimpeded height in the upwardtrajectory.
 22. The apparatus of claim 13, wherein the cooling drumincludes a first cylinder, having an open top end and an open bottom endand surrounding a gaseous medium; a second cylinder, coaxial with thefirst cylinder and surrounding the first cylinder, and having a top endthat is closed around the top end of the first cylinder, and a bottomend that is closed around the bottom end of the first cylinder, forminga reservoir between the first and second cylinders; and a system forcontrolling the temperature of the gaseous medium.
 23. The apparatus ofclaim 22, wherein the system for controlling the temperature of thegaseous medium includes a first fluid inlet, disposed in an outer wallof the second cylinder, that receives a first fluid to be stored in thereservoir.
 24. The apparatus of claim 23, wherein the system forcontrolling the temperature of the gaseous medium includes a secondfluid inlet, disposed in the outer wall of the second cylinder, forreceiving a second fluid to be dispersed within the first fluid in thereservoir.
 25. The apparatus of claim 24, wherein the system forcontrolling the temperature of the gaseous medium includes a dispersaltube, connected to the second fluid inlet and surrounding the firstcylinder within the reservoir, that receives the second fluid throughthe second fluid inlet, wherein the dispersal tube includes a pluralityof holes through which the second fluid is dispersed within the firstfluid.
 26. The apparatus of claim 25, wherein the dispersal tube is acircular closed loop for receiving the second fluid from the secondfluid inlet and for dispersing the second fluid into the first fluid,within the reservoir around the first cylinder, through the plurality ofholes.
 27. The apparatus of claim 13, further comprising a gas screendisposed between the buffering chamber and the cooling drum, whichprovides temperature separation between respective media in thebuffering chamber and the cooling drum.
 28. The apparatus of claim 27,wherein the gas screen includes a hollow disk having a top face with anopening for receiving the droplet from the buffering chamber, a bottomface with an opening for providing the droplet to the cooling drum, andcircular outer wall connecting the top and bottom faces; and a fan,disposed within the hollow disk and positioned such that it blows afluid medium within the hollow disk in a direction that is tangential tothe outer wall.
 29. The apparatus of claim 14, wherein the collectorarrangement includes a reservoir that holds a liquid into which themetal sphere falls after passing through the cooling drum; a pipe,connected to a bottom end of the reservoir and in fluid communicationwith the reservoir, that receives the metal sphere and a volume of theliquid from the reservoir; and a delivery system that delivers the metalsphere to a collection basket.
 30. The apparatus of claim 29, whereinthe reservoir has lower sides that slope toward an opening in the pipe.31. The apparatus of claim 29, wherein the pipe is an elbow joint havinga bend in which the metal sphere settles.
 32. The apparatus of claim 29,wherein the delivery system is a pump that pumps the metal sphere andthe volume of the liquid to the collection basket; and the collectionbasket is located at a level that is higher than a level of the liquidin the reservoir.
 33. The apparatus of claim 32, wherein the collectorarrangement further includes a holding tank in which the collectionbasket is disposed; and the collection basket has openings that aresmaller than the metal sphere, through which the volume of liquid pass.34. The apparatus of claim 33, wherein the collector arrangementincludes a return channel, in fluid communication between the holdingtank and the reservoir, by which liquid passing through the openings inthe collection basket is returned to the reservoir.
 35. The apparatus ofclaim 14, wherein the cooling drum is a plurality of cooling drums,including a first cooling drum, disposed to receive the droplet from thebuffering chamber; and a last cooling drum disposed to provide the metalsphere to the collector arrangement.